In an attempt to reduce the price of solar energy, many developments have been made with respect to lowering the cost of precisely repositioning and calibrating a surface with two degrees of freedom. In concentrated solar thermal systems, heliostat arrays utilize dual axis repositioning mechanisms to redirect sunlight to a central tower by making the normal vector of the heliostat mirror bisect the angle between the current sun position and the target. Heat generated from the central tower can then be used to generate steam for industrial applications or electricity for the utility grid.
Concentrated photovoltaic (CPV) systems take advantage of dual axis mechanisms in order to achieve a position where the vector normal to the CPV surface is coincident with the solar position vector. When the CPV surface is aligned to the sun, internal optics are able to concentrate sunlight to a small, high efficiency photovoltaic cell.
Dual axis positioning systems also enable flat plate photovoltaic (PV) systems to produce more power through solar tracking. Compared to fixed tilt systems, dual axis PV systems produce 35-40% more energy on an annualized basis. While this increase in energy production may seem attractive, current technology marginalizes the value of biaxial solar tracking by increasing total system capital and maintenance costs by 40-50%.
Traditional solutions to the problem of controlling and calibrating an individual surface fall into one of three main categories: active individual actuation, module or mirror ganging, and passive control. In the active individual actuation model, each dual axis system requires two motors, a microprocessor, a backup power supply, field wiring, and an electronic system to control and calibrate each surface. Moreover, all components must carry a 20+ year lifetime and the system needs to be sealed from the harsh installation environment. In an attempt to spread out the fixed cost of controlling an individual surface, conventional engineers' thinking within the individual actuation paradigm are building 150 square meters (m^2) heliostats and 225 square meters PV/CPV trackers. While control costs are reduced at this size, large trackers suffer from increased steel, foundational, and installation requirements.
Another approach attempts to solve the fixed controls cost problem by ganging together multiple surfaces with a cable or mechanical linkage. While this effectively spreads out motor actuation costs, it places strict requirements on land grading, greatly complicates the installation process, and incurs a larger steel cost due to the necessary stiffness of the mechanical linkages. Due to constant ground settling and imperfections in manufacturing and installation, heliostat and CPV systems require individual adjustments that increase system complexity and maintenance cost.
Passive systems utilizing hydraulic fluids, bimetallic strips, or bio-inspired materials to track the sun are limited to flat plate photovoltaic applications and underperform when compared to individually actuated or ganged systems. Moreover, these systems are unable to execute backtracking algorithms that optimize solar fields for energy yield and ground coverage ratio.